That's a great question, I'm glad you asked!
Constant Contact has a strict policy against selling, sharing, or stealing contacts. In order to use our services our customers must accept our terms which include that they have permission to mail to their contacts, and we do not allow anyone to use our service with purchased or stolen lists. We do not use your contacts in any way other than to provide you the service so that you can mail to them. You can take a look at our privacy policies here.
I hope that helps answer your question!